FUNSEX-DEPHYND · The functional significance of sex and death in phytoplankton differentiation

The functional significance of sex and death in phytoplankton differentiation

Marine phytoplankton is a key element in global biogeochemical cycles. These organisms are known to exhibit complex life cycles alternating between prolonged phases of mitotic division and brief phases of sexual reproduction, involving meiosis, syngamy and large changes in cell properties. These changes are poorly characterised and their ecological significance remains mysterious (refer to von Dassow and Montresor, 2011). This project studied the differentiation between the 1N and 2N phases of coccolithophorids under conditions of exponential growth and in response to stress, combining genomic and transcriptomic-based techniques with physiological experiments. The transcriptomes of exponentially growing 1N and 2N cells of three coccolithophore species, emiliania huxleyi, gephryocapsa oceanica and coccolithus braarudii were characterised by deep Sanger sequencing, including more than 38 000 expressed sequence tags (ESTs) in total from each species, as well as by 454 sequencing for e. huxleyi. The following results were obtained: 1. coccolithophore transcriptomes were estimated to contain more than 20 000 expressed genes, a high gene number for single-celled organisms 2. approximately 50 % of the transcribed genes might be differentially expressed between 1N and 2N cells 3. only 2N cells of e. huxleyi and g. oceanica were calcified. Many genes, potentially relating to calcification and pH balance in coccolithophores, were identified based on homology and highly 2N-specific expression patterns 4. the conserved components of flagella were identified based on homology and highly 1N-specific expression patterns. Sanger-sequencing results for e. huxleyi were published by von Dassow et al., 2009. Selected genes, identified as possibly relating to bi-carbonate and pH balance, were investigated in a study of the response of e. huxleyi to increased pCO2, as defined by Richier et al., 2010. Further characterisation of other genes was ongoing, by the time of the project completion, in the laboratory of collaborator Dr Glen Wheeler. G. oceanica and c. braarudii transcriptomes were planned to be published in 2011. Experiments comparing physiological and transcriptomic responses of 1N and 2N cells to starvation for nitrogen (N) and phosphorus (P) were performed using microarrays with the following observations: 1. 1N cells of e. huxleyi and g. oceanica grew faster than 2N cells under non-stressed conditions 2. efficiency of photosynthetic electron transport (Fv/Fm) of 1N cells declined rapidly under N starvation. In contrast, Fv/FM of 2N cells declined rapidly under P starvation 3. genes whose expression changed between 1N and 2N cells under N-starvation versus P-starvation was identified. The finding that 1N cells appeared to be more tolerant to P-starvation than 2N cells was interesting as blooms of 2N cells of e. huxleyi were often associated with low P conditions. Results were to be published in 2011. Genome-wide comparisons of e. huxleyi strain RCC1216/1217 to strain CCMP1516 identified large genomic variation among e. huxleyi strains. Genes for highly conserved flagellar components were selectively lost from the CCMP1516 genome. A survey of more than 70 other strains revealed that isolates from warm waters tended to lose the ability to form flagellated 1N cells, whereas strains from temperate coastal waters maintained the full life cycle. This suggested that haplo-diploid life cycles were maintained in dynamic environments with large temporal and spatial variability. Results were planned to be published in 2011 and led to the sequencing of the entire genome of strain RCC1217 by the collaborator Dr Thomas Mock from the University of East Anglia and the genome analysis centre of the United Kingdom. Dr von Dassow collaborated to investigate genomics and evolution of life cycles of photosynthetic eukaryotes, identifying conserved meiotic genes in fully sequenced genomes of diatoms, prasinophytes and brown algae (refer to Bowler et al., 2008; Worden et al., 2009; Cock et al., 2010). Finally, the demonstration of polyploidisation in laboratory diatom populations (von Dassow et al., 2008) was extended. It occurred that polyploidisation might function in on-going speciation of natural diatom populations (refer to Koester et al., 2010).

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